I plan to travel to France this summer and rent a car for my convenience. The trip might last 12 days. However, I don't want to buy a GPS with France map nor rent a GPS because this cost too much and I may never use it again afterwards.

I have a Garmin GPS only with US maps, I am not sure if I can install the France map with a cost less than $10. Hence, I was wondering if I can download a GPS map for my android smartphone? If possible, where/which app should I download?

If you're going through France with a car and GPS, remember to disable your radar detector. A few years ago, their government created a law which forbids GPS devices and similar to show the driver the exact position of radar devices (specifically the ones that give tickets). They may say that in the next X yards there's a radar, but not where exactly it is.
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Nate KerkhofsApr 28 '14 at 12:40

+1 for OsmAnd. It provided excellent offline navigation and downloading the map areas is easy. The search is not as good as Google Maps, so you might consider simply getting a French SIM card (from Lebara, for example) and using that.
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Walter NissenApr 30 '14 at 2:47

You have a garmin gps. If it has support for a micro sd card you might be lucky. Open Street Map provides community contributed maps. it is possible to convert these maps to the Garmin image fill format and load it to your Garmin unit. To do this you need to be able to access the internal memory space of your GPS unit. I have a Garmin Vista HCx for quite some time now, which has a micro sd card slot and I now have a batch of micro sd cards with maps from all over the world. Whenever I go somewhere I simply slide the relevant sd-card into the unit and I am ready to explore. The recipe is relatively simple, you need to make the gmapsupp.img file containing the maps and load that file to your sd card under the Garmin folder. You can download precompiled gmapsupp.img files, or you can compose your own selection.

My GPS unit does not support voiced instructions, so I don't know how well this works with instructions. I have used my Garmin handheld unit in France though and I managed quite well. I have to admit that as an additional asset I do have the Michelin France maps on my iPad. I do miss the "helicopter-view" for oversight in a car navigation unit, so I like having access to digital maps on a iPad screen.

I have used MapFactor, which was recommended to me on this site. It's free, uses the OpenStreetMap database (also free, with free updates) and is quite usable. You also pre-download maps so don't have to use 3G downloading as you drive around.

However if your GPS has country-specific maps for less than $20 I personally would opt for that. I find GPSes (or at least our GPS) easier to use for long trips than phones due to better battery life and their specifically designed software.

Mappy GPS Free may help for offline navigation. In the French store, the app, the map of France and one French voice are all free. Other countries and voices can be purchased separately for a fee. From other stores, options, pricing and availability may be different... Map data is licenced from TomTom.